The present invention relates to a liquid crystal image display for displaying video with driving liquid crystal, in particular, relates to a technology being suitable to be applied to a low price and high performance liquid crystal image display using a thin film transistor (TFT) therein.
The structure of a video display device of the conventional art is shown in FIG. 7.
In a matrix-like manner are disposed pixels, each being constructed with a TFT switch 101 and a pixel capacitor 102 which has a pixel electrode connected to a source electrode thereof as well as a common electrode. A liquid crystal is provided in the pixel capacitor 102 at a predetermined position thereof, and it is changed or modulated in the optical characteristic thereof by write-in voltage to the pixel capacitor 102, thereby enable to display the video thereon. To the gate of the TFT switch 101 is connected a gate line 103, and at one end of which is provided a vertical shift register 105. Also, the drain of the TFT switch 101 is connected a signal line 104, and at one end of which is provided a DA converter 106. On a while, the signal line 109 is inputted through a signal latch 107 to the DA converter 106. To the signal latch 107 is inputted a signal of a horizontal shift register 110. The common electrodes of all the pixels are connected into one body to which a constant voltage is applied.
Further, herein the each portion, such as the DA converter 106, etc., shown in FIG. 6 is constructed by using Poly-Si (polycrystalline silicon) TFT.
Hereinafter, operation of the present conventional art will be explained. A digital input signal inputted to the signal line 109 is latched in the signal latch 107, sequentially, in accordance with scanning of the horizontal shift register 110. The input signals being latched are inputted, collectively, to the DA converter 106 so as to be converted into analogue signals to be applied to the signal lines 104. At this moment, in the pixels on a line whose gate line 103 is selected by this vertical shift register 105, since the TFT switch 101 thereof is in ON state, the analogue signal which is applied to the signal line 104 is written into the pixel capacitor 102. As a result of this, an electric field corresponding to the input signal is applied to the liquid crystal portion of the pixels, into which the signals are written, therefore it is possible to display the video depending upon the signals.
An example of such the conventional video display device is described in detail, for example in xe2x80x9cSociety for Information Display International Symposium Digest of Technical Papers 96 (SID 96), pp.21-24xe2x80x9d, etc.
Also, in Japanese Patent Laying-Open No. Hei 6-266318 (1994) is disclosed a technology, that the common electrodes are provided with being divided for each of the signal lines, and a signal voltage and a voltage having a reversed polarity thereof are applied in synchronism with a signal which is supplied to the signal line, thereby suppressing the voltage between anodes of field effect transistors within the pixels.
In the former of the above-mentioned conventional arts, since the DA converter 106 is not connected to the signal line 104 directly, there is a problem that an output of the DA converter 106 is modulated by a load carrying capacitor of the signal line 104 if an output impedance of the DA converter 106 is not designed to be small sufficiently. While trying to design the output impedance of the DA converter 106 to be sufficient small, an area of the DA converter 106 comes to be large, excessively.
For preventing such things, in a driver circuit using mono-crystalline Si (silicon) transistors, generally, a buffer circuit is provided between the output of the DA converter 106 and the signal line 104. However, in a case where the pixel portion is formed with the driver circuit including peripheral circuits thereof in one body, by using Poly-Si (polycrystalline silicon) TFT, it is very difficult to provide the buffer circuit. This is because the Poly-Si TFT, differently from Si (silicon) transistor, inherently has very large unevenness or variation in the threshold voltage thereof, when the buffer circuit is provided for each of the lines, a large fixed pattern noise is caused on the display video due to the unevenness or variation of the threshold voltage for each line.
Also, in the above-mentioned Japanese Patent Laying-Open No. Hei 6-266318 (1994), there is disclosed that the common electrode is provided separately for each signal line and a signal voltage and a voltage of reversed polarity are applied to the common electrode in synchronism with the signal supplied to the signal line, however there is not disclosed the structure of removing the fixed pattern noise due to the unevenness or variation the threshold voltages of elements which construct a signal voltage applying portion, such as the buffer circuit, etc., at all.
An object, according to the present invention, is to provide a liquid crystal image display for high quality video, with removing the fixed pattern noise due to the unevenness or variation in the threshold voltage of the elements which construct the signal voltage applying portion, such as the buffer circuit, etc.
The object mentioned above and other objects, as well as the novel features, according to the present invention, will be apparent from the description and the attached drawings of the specification.
Explaining an outline of the representative example of the present invention being disclosed herewith, briefly, it is as follows.
Namely, according to the present invention, for achieving the above-mentioned object, the common electrodes are provided independently for each signal line, and an output of a signal voltage applying portion, such as the buffer circuit, etc., is connectable to both the corresponding common electrode and the signal line, selectively, thereby enabling to apply voltage to both the signal line and the common line by means of the signal voltage applying portion, such as the buffer circuit, etc. The variation of the threshold voltages in the signal voltage applying portions, such as the buffer circuits, etc., each of which is provided for each of lines, appears to be the variation of offset voltages at outputs of signal applying portions, however with application of such the construction mentioned above, according to the present invention, the offset voltage is supplied to the both of the corresponding signal line and the common electrode, equally, within one of the lines, therefore no variation of the threshold voltages can be observed between the lines among the pixel electrodes which drive the liquid crystal and the common electrodes. Accordingly, the large fixed pattern noises, being caused due to the variation of the threshold voltages among elements of the signal voltage applying portion for each line, will not occur on the display video, thereby providing a liquid crystal image display having a high quality.